Immediate implant loading produces micromotion at the bone-implant interface, which can lead to fibrous or fibrocartilaginous encapsulation and implant failure. While mechanical conditions such as these appear to influence healing of skeletal tissues, there is only an incomplete understanding of the key physical factors and underlying mechanisms that influence cell fate decisions at the bone-implant interface. As a result, there is a limited scientific basis for the design of load-bearing implants for use in skeletal tissues. Our working hypothesis is that deformation in healing tissue as defined by principal strain magnitude, spatial extent, and related factors regulates cell fate decisions at the bone-implant interface. The proposed work will test this hypothesis in a mouse model using a miniature implant device that permits control and quantification of the biomechanical environment (i.e., the strain state) of the healing bone-implant interface, and which allows us to measure the cellular response to different mechanical stimuli using molecular, cellular, and genetic approaches. Aim 1 will examine how variations in strain magnitude, spatial extent, and number of strain cycles per day influence mechanobiology at the healing bone-implant interface. Our miniature device and implant design allows us to create situations where either principal compressive strain is the dominant type of strain, or principal tensile strain is the dominant type of strain. Strain will be measured using micro-CT and digital image correlation then correlated, in time and space, with the in vivo cellular response. Together, these experimental results will allow us to identify regimes of "safe", "dangerous", or "stimulatory" principal tensile or compressive strain. Aim 2 will explore how cells at a healing interface detect and decipher physical stimuli such as strain. Primary cilia are implicated in fetal bone mechanotransduction;we hypothesize that primary cilia are required for implant mechanotransduction. We will test this hypothesis by conditionally inactivating an essential component of the primary cilium molecular machinery, Kif3a, and measuring how this alters response to strain state and healing of the bone-implant interface. Aim 3 will test whether biomechanical strain operates at multiscale levels (macro, micro, and nano) to influence cell differentiation at implant interfaces. We hypothesize that an implant's surface texture (roughness) can influence local strain fields at the same size scale, e.g., microns. This will be tested by subjecting implants with differing surface textures to micromotions that are of the same size as the roughness. Overall, this project will contribute scientific insight that can guide design decisions with skeletal implants. PUBLIC HEALTH RELEVANCE: Loosening and failure of load-bearing bone implants remain a major health problem. In order to help solve this problem, this project is trying to understand why certain mechanical conditions at the bone-implant interface are dangerous to the healing processes, while others are safe and sometimes even beneficial to healing. Our work focuses on the role of deformation of healing tissues;the ways that living cells sense that deformation;and the ways that an implant's overall shape as well as its surface roughness can be better designed to take advantage of this knowledge.